In recent years, rapid development of semiconductor technology has led to rapid progress of a reduction in size of semiconductor packages, the adoption of multipin, the adoption of fine pitch, minimization of electronic components and the like. That is, the semiconductor field has entered the so-called “age of high density packaging.” Regarding printed wiring boards, this has also led to a change from single side wiring to double side wiring and, in addition, the adoption of a multilayer structure and a thickness reduction (Iwata and Harazono, “Denshi Zairyo (Electronic Material),” 35 (10), 53 (1996)).
Pattern formation methods used in the formation of such wiring and circuits include: a method which comprises etching a metal layer, provided on a substrate in a layer construction of metal layer-insulating layer-metal layer, with an acidic solution, such as a ferric chloride solution, to form wirings, then subjecting the insulating layer, for example, to dry etching such as plasma etching or laser etching, or wet etching such as etching with hydrazine, to remove the insulating layer to form a desired shape for layer-to-layer continuity purposes (Japanese Patent Laid-Open No. 164084/1994), and connecting the wirings to each other, for example, through plating or electrically conductive paste; and a method (Proceedings of the 7th Symposium of Japan Institute of Electronics Packaging, issued in 1999) which comprises providing an insulating layer in a desired form using a photosensitive polyimide (Japanese Patent Laid-Open No. 168441/1992) or the like and then plating gaps to form wiring.
A tendency toward downsizing of electric products in recent years has led to a reduction in thickness of each layer constituting metal layer-polymeric insulating layer, and these layers each are in many cases used in a thickness of not more than 100 μm. When wiring has been formed of such thin layer, a warpage disadvantageously takes place in wiring due to a difference in coefficient of thermal expansion between the metal layer and the polymeric insulating layer.
When the thermal properties of the insulating layer and the conductor layer are known, the warpage σ of this substrate can be calculated according to the following equation (Miyaaki and Miki, NITTO TECHNICAL REPORT, 35 (3), 1 (1997)).
  σ  =                    31        ⁢                                  ⁢                  E          1                ⁢                  E          2                            2        ⁢                                  ⁢                  h          ⁡                      (                                          E                1                2                            +                              14                ⁢                                                                  ⁢                                  E                  1                                ⁢                                  E                  2                  2                                            +                              E                2                2                                      )                                ⁢    ΔαΔ    ⁢                  ⁢    T  wherein
E1: modulus of the metal,
E2: modulus of the insulating layer,
Δα: difference in coefficient of thermal expansion between the metal and the insulating layer,
ΔT: temperature difference, and
h: layer thickness 1: wiring length.
According to this equation, the following two methods are considered effective for reducing the warpage of wiring:
1. a reduction in modulus of insulating layer; and
2. a reduction in the difference in coefficient of thermal expansion between the insulating layer and the metal wiring layer.
Regarding the wiring formation method, in the laminate used in the method for the formation of wiring through etching of a metal in the laminate having layer construction of first metal layer-insulating layer-second metal layer, in order to reduce the warpage of the laminate, a low-expansion polyimide is used as the insulating layer from the viewpoint of the necessity of rendering the coefficient of thermal expansion of the metal identical to the coefficient of thermal expansion of the insulating layer (U.S. Pat. No. 4,543,295 and Japanese Patent Laid-Open Nos. 18426/1980 and 25267/1977).
Since, however, the low-expansion polyimide is not generally thermoplastic, the adhesion to metal layers is poor making it difficult to provide adhesive strength high enough to withstand practical use. To overcome this problem, a thermoplastic polyimide resin or epoxy resin having good adhesion to the metal layer is used as an adhesive insulating layer between the metal layer and the insulating layer (core layer) of the low-expansion polyimide (Japanese Patent Laid-Open No. 58428/1995).
Since the thermoplastic resin generally has a high coefficient of thermal expansion, the lamination onto a metal is causative of the warpage. To overcome this drawback, the thickness of the low-expansion core insulating layer having a coefficient of thermal expansion close to that of the metal is made larger than the thickness of the adhesive layer to avoid the appearance of warpage of the whole laminate on the surface of the laminate. The smaller the thickness of the adhesive insulating layer, the better the warpage preventive effect. When the thickness of the adhesive insulating layer is excessively small, however, the adhesion is deteriorated. At least when the total thickness of the adhesive layers respectively overlying and underlying the core layer is not more than the half of the thickness of the core layer, the warpage is less likely to occur. For this reason, for commercially available laminates fabricated for electronic circuit components, in many cases, the total thickness of adhesive insulating layers is not more than the half of the thickness of the core insulating layer. The formation of the adhesive insulating layer in a smallest possible thickness, which can ensure the adhesion, is regarded as ideal (Japanese Patent Laid-Open No. 245587/1989).
At the present time, rapid expansion of production of personal computers has lead to increased production of hard disk drives incorporated in the personal computers. A component, in the hard disk drive, called a “suspension,” which supports a head for reading magnetism, is being shifted in its main products from one, wherein copper wiring is connected to a stainless steel plate spring, to one called a “wireless suspension” comprising copper wiring which has been connected directly to a stainless steel plate spring, from the viewpoint of coping with the size reduction.
The wireless suspension is mainly prepared using a laminate having a layer construction of first metal layer-adhesive insulating layer-core insulating layer-adhesive insulating layer-second metal layer. An example of the laminate is such that the first metal layer is formed of a copper alloy foil, the second metal layer is formed of a stainless steel foil, and the insulating layer is comprised of a core insulating layer and an adhesive insulating layer provided on both sides of the core insulating layer. A wireless suspension using the laminate is scanned on a disk being rotated at a high speed and thus is a member to which fine vibration is applied. Therefore, the adhesive strength of wiring is very important. Accordingly, the wireless suspension using the laminate should satisfy strict specifications.
Hard disk drives are devices for recording information thereon. Therefore, a high level of data read/write reliability is required. To meet this requirement, dust and outgas produced from the wireless suspension should be minimized.
A component called the “wireless suspension” is produced mainly by two methods, an additive method wherein wiring is formed by plating, and a subtractive method wherein wiring is formed by etching a copper foil. In the case of the subtractive method, plasma etching by dry process is solely used for patterning of polyimide as the insulating layer.
In the laminate such as the three-layer material used in electronic components, in general, in order to render the coefficient of thermal expansion of the conductive inorganic material layer and the coefficient of thermal expansion of the insulating layer identical to each other for preventing the warpage of the substrate, a low-expansion insulating layer, particularly a low-expansion polyimide, is in many cases included. In the laminate in which only a low-expansion polyimide is used in the insulating layer, polyimide films, for example, KAPTON (tradename; manufactured by Du Pont-Toray Co., Ltd.), Upilex (tradename; manufactured by Ube Industries, Ltd.), and APIKAL (tradename; manufactured by Kanegafuchi Chemical Ind. Co., Ltd.), are used as the low-expansion polyimide. A metal layer (mainly copper) is formed, for example, by sputtering or electroless plating on the surface of the low-expansion polyimide film, and, thereafter, the thickness of the conductor layer is increased by electroplating (hereinafter referred to as “type-I laminate”). Another type of laminate is such that an insulator having a three-layer structure, wherein a layer of an adhesive (for example, an epoxy adhesive) other than polyimide is formed on both sides of a low-expansion polyimide, that is, having a layer construction of adhesive other than polyimide-low-expansion polyimide-adhesive other than polyimide, is provided and a conductor foil is adhered to the insulating layer by thermocompression bonding (hereinafter referred to as “type-II laminate”). A further type of laminate is such that an insulator having a three-layer structure, wherein an adhesive polyimide layer is formed on both sides of a low-expansion polyimide, that is, having a layer construction of adhesive polyimide-low-expansion polyimide-adhesive polyimide, is provided and a conductor foil is adhered to the insulating layer by thermocompression bonding (hereinafter referred to as “type-III laminate”).
In the type-I laminate, since the insulating layer is formed of a polyimide having a single composition, the warpage is less likely to occur. Further, a thin metal layer can be formed. Therefore, this is advantageous for the formation of fine wiring. On the other hand, in the type-II laminate and the type-III laminate, since the conductor layer is formed by thermocompression bonding, the conductive layer can be selected from various types. For example, the preparation of a laminate using a rolled copper foil, a stainless steel foil or the like is possible. The type-II laminate advantageously has good adhesion. For the type-III laminate, since the adhesive layer is formed of highly heat-resistant polyimide, the heat resistance is advantageously good. Further, both the type-II and type-III laminates have an additional advantage that the thickness of the metal layer can be increased.
Since spring properties are required of the wireless suspension, a stainless steel foil is in many cases used as the metal layer. An example of the laminate structure is copper foil-adhesive polyimide low-expansion polyimide-adhesive polyimide-stainless steel. In the conventional wireless suspension, since the etching area of the insulating layer is large, rather than laser beam etching, plasma etching, which belongs to the same category of process, i.e., dry process, is mainly used for patterning of the insulating layer. In the plasma etching, however, the etching rate is low, and, thus, the time necessary for etching is long. Further, since sheet-by-sheet production is adopted, the productivity is low. Moreover, the apparatus for plasma etching is so expensive that the production cost is disadvantageously very high.
For the above reason, patterning of the insulating layer by a wet process, which is high in etching rate and thus is high in productivity and can realize low apparatus cost, has been desired in the art.
Also in electronic components, such as flexible printed boards or multilayer substrate, wherein a hole is formed by laser beams for providing continuity between layers in the multilayer substrate followed by pattern drawing in a mold into a desired form, when the wet etching technique is used, the step of hole formation and the step of pattern drawing can be simultaneously carried out. In addition, a fine shape, which cannot be formed in the mold, can be formed by wet etching. Therefore, patterning of the insulating layer by the wet process has also been desired in each field of electronic components.
For the type-II laminate, the use of an epoxy adhesive poses a problem that the solvent resistant is so high that wet etching cannot be carried out at all.
For the type-III laminate, due to a significant difference in etching properties between the adhesive polyimide layer and the low-expansion polyimide layer, the sectional form after etching is not sharp, and this makes it substantially difficult to prepare electronic components by wet etching.
For the type-I laminate, in some cases, wet etching is adopted. Since, however, the conductive inorganic material layer is formed by sputtering or the like, a metal is collided at a high speed against the surface of the polyimide. As a result, the metal bites into the surface layer, as well as into the inside of the polyimide layer. This somewhat denatures the polyimide in the surface layer. The adhesion between the insulating layer and the conductive inorganic material layer in the type-I laminate relies mainly upon a chemical bond or chemical interaction between the conductive inorganic material layer and the insulating layer. Therefore, the affinity of the conductive inorganic material layer for the insulating layer is high. This poses a problem that, when the type-I laminate is wet etched, the insulating layer in its denatured portion located at the interface of the conductive inorganic material layer and the insulating layer remains unetched, resulting in pattern detects.
On the other hand, in the laminate prepared by integrating the conductive inorganic material layer with the insulating layer by pressing, as compared with the chemical bond or chemical interaction, the anchor effect attained by concaves and convexes on the surface of the conductive inorganic material layer more greatly contributes to the adhesive strength. Therefore, an unfavorable phenomenon, wherein a portion to be etched remains unetched, is less likely to occur. As described above, in the production of the laminate by pressing, since the degree of freedom in the selection of the conductive inorganic material layer is large, products, which could not have been produced in the case of the formation of the conductive inorganic material layer by sputtering, can also be produced.
Polyimides generally have poor solubility in solvents. Since, however, they are decomposed by a hydrazine or alkali solution, various studies have hitherto been made on wet etching of polyimide films with a chemical liquid. For example, Japanese Patent Laid-Open No. 4577/1975 discloses a production process of a wiring structure using hydrazine and ammonia. Japanese Patent Laid-Open No. 103531/1983 discloses a method for etching a polyimide film with an inorganic basic aqueous solution. Japanese Patent Laid-Open No. 65727/1982 discloses a method for etching a polyimide with an aliphatic diamine. Other methods for wet etching a polyimide disclosed up to now are such that a chemical liquid prepared by mixing water or an organic polar solvent with hydrazine/inorganic alkali/organic alkali/aliphatic amine (diamine)/aliphatic alcohol as a solvent is used (for example, Japanese Patent Laid-Open Nos. 74041/1983, 96632/1983, 101228/1991, 190610/1993, 202206/1993, and 157560/1995).
Hydrazine as a component for decomposing the polyimide, however, is highly toxic and thus is unsuitable for use in production process. For this reason, in proposals in recent years, in many cases, an etching solution comprising an inorganic basic aqueous solution and various additives added thereto is used.
Conventional methods for etching a polyimide film by wet etching to form a pattern include: a method wherein a metal is used in a pattern mask (Japanese Patent Laid-Open No. 283486/1993); a method wherein a solvent development-solvent separation-type negative-working liquid resist is used (Japanese Patent Laid-Open No. 301981/1993); and a method wherein a solvent development-solvent separation-type positive-working liquid resist is used (Japanese Patent Laid-Open Nos. 27464/1976, 49068/1978, 49068/1978, 65727/1982, and 74041/1983). These conventional methods for wet etching a polyimide film to form a pattern is effective in shorting the time necessary for patterning of the insulating layer.
The laminate using a polyimide as an insulating layer is in many cases thin and thus has low rigidity. Therefore, this laminate is disadvantageously inferior in handleability to conventional rigid glass epoxy substrates or the like. This is a serious limitation on process design.